1. Field of the Invention
The present invention generally relates to an apparatus for cooling bearings rotatably supporting a spindle of a machine. More particularly, the present invention relates to an apparatus for cooling rolling-contact type bearings for supporting a spindle, especially a spindle to be rotated at a high speed. The apparatus for cooling the high speed spindle bearings according to the present invention may be used with spindle bearings of, e.g., a machine tool such as a lathe, a milling machine, and a machining center, and various turbine shafts.
2. Description of the Related Art
For example, a spindle of a machine tool is rotatably supported by rolling-contact type bearings including rolling elements therein, and can be rotated at a high speed. Therefore, the higher the rotational speed of the spindle, as well as the larger a cutting load applied to the spindle bearings, the greater an amount of heat generated by the spindle bearings. Such an increase in the amount of heat generated by the spindle bearings causes a thermal deformation of the spindle, and thus reduce the accuracy of the machining by the machine tool, and will often lead to a seizing of the bearings. Accordingly, the spindle bearings supporting a spindle of a machine to be rotated at a high speed must be cooled during the rotation of the spindle.
One typical known method of cooling a bearing mounted in a housing, for rotatably supporting a shaft or a spindle, is carried out in a manner such that a flow of a liquid coolant is fed to the housing at positions adjacent to the outer race of the bearing, to thereby cool the bearing at the outer periphery thereof. In another known method, a liquid lubricant in the form of a liquid jet is directly spouted toward the ball bearings or roller elements and the track of the bearing, to both cool and lubricate the bearing. In these known methods, although the outer races of the bearings are effectively cooled, the inner races of the bearings often cannot be cooled, and accordingly, a problem occurs in that a thermal expansion of the outer race is different from that of the inner race of the same bearing. To solve this problem, different bearing cooling methods have been proposed, and recently adopted, to supply bearing balls or roller elements of a bearing with a liquid lubricant radially inside the bearing. Namely, in one of these methods, a flow of a liquid coolant is fed to interior of a rotating shaft or spindle supported by a bearing, to intentionally cool the inner race of the bearing, via the shaft or spindle. In another method, a liquid lubricant is supplied into an axial bore of a shaft or spindle, and the flow of lubricant is then fed from the axial bore into radial passageways which are formed to run radially through the shaft or the spindle and the inner race of the bearing, so that the lubricant reached the ball bearings or roller elements after passing through the inner race, and thus the liquid lubricant lubricates and cools the bearing from the inside thereof. Japanese Unexamined Patent Publications ( Kokai ) Nos. 63-231021 and 2-224945 disclose a spindle bearing cooling apparatus, respectively, which is constructed and operated in accordance with the principle of the above-mentioned method, i.e., the method of an intentional cooling of the inner race of a bearing.
In the apparatus of Japanese Unexamined Patent publication ( Kokai ) 2-224945, a part of a flow of a liquid lubricant supplied into the spindle is fed through the spindle at a part thereof located adjacent to the inner races of a pair of spindle bearings, and collected through an oil discharge passageway formed in a housing in which the spindle bearings are seated. The other part of the liquid lubricant is not permitted to flow through the part of the spindle adjacent to the inner races of the bearings, but is fed through an oil passageway formed in a portion of the housing far from the spindle housings and collected in an oil jacket radially spaced from the outer races of the spindle bearings, and then fed through another oil discharge passageway formed in a sleeve element arranged around the housing, to thereby apply a cooling effect to the outer races of the spindle bearings. Therefore, the temperatures of the part of the liquid lubricant cooling the inner races of the bearings and that of the part of the liquid lubricant cooling the outer races of the spindle bearings are substantially the same. Nevertheless, although heat generated by a spindle bearing is usually transmitted toward the outside thereof and easily radiated from the outer race of the spindle bearing, the heat is not easily radiated from but stays inside the inner race thereof, and thus the temperature of the inner race of the bearing is higher than that of the outer race. Accordingly, an expansion of the inner race is larger than that of the outer race, and as a result, the ball bearings or roller elements are subjected to a compression force which leads to an increase in a resistance of the races to the rotation of the ball bearings or roller elements. Therefore, this increase in the heat generated by the spindle bearing shortens the operating life of the spindle bearing or cause a seizing thereof. To solve this problem, a spindle bearing mounted in a housing for rotatably supporting a spindle fitted in the inner race thereof must be cooled in such a manner that the temperature of the inner race thereof is kept lower than that of the outer race, during rotation of the spindle. Nevertheless, Japanese Unexamined Patent Publication 2-224945 neither suggests nor teaches the necessity of keeping the temperature of the inner race lower than that of the outer race of the spindle bearing.
In the apparatus for lubricating a rolling bearing, as disclosed in Japanese Unexamined Patent Publication ( Kokai ) No. 63-231021, a liquid lubricant is introduced into axial grooves formed in the outer circumference of a spindle, and the lubricant is then supplied to the rolling elements, such as bearing balls, via radial grooves formed in the opposite end faces of the inner race of the spindle bearing. Therefore, when the liquid lubricant flows through the radial grooves of the inner race of the spindle bearing, a cooling of the inner race is carried out by the liquid lubricant. Nevertheless, the cooling of the inner race of the spindle bearing of this apparatus is unsatisfactory because when heat is generated by the spindle bearing, the heat is mainly transmitted from an entire cylindrical inner face of the inner race of the spindle bearing to an entire cylindrical outer face of the spindle, due to the contact of the inner cylindrical face of the inner race with the entire cylindrical outer face of the spindle. Nevertheless, the flow of the liquid lubricant in the radial grooves of the inner race cannot be brought into contact with an entire cylindrical face of the inner race, and the liquid lubricant flowing in the axial grooves of the spindle is not able to directly remove heat from and cool an entire cylindrical surface of the spindle.
Accordingly, the spindle bearing cooling technique known from Japanese Unexamined Patent Publications (Kokai ) Nos. 2-224945 and 63-231021 are unsatisfactory from the viewpoint of the cooling of a spindle bearing by a liquid lubricant, and accordingly, an improvement thereof is required.
Furthermore, in the above-mentioned spindle bearing method of cooling by a liquid lubricant supplied from radially inside the inner race, a large amount of liquid lubricant must be supplied to apply an even cooling effect to the entire circumference of the spindle bearing. Namely, a very large amount of liquid lubricant must be supplied into a cavity of a spindle, i.e., an axial bore of the spindle supported by the bearing, so that the liquid lubricant flows out of the cavity toward the bearing through many hole-like radial passageways of the spindle formed therein, and through many corresponding radial passageways formed in the inner race of the spindle bearing. Nevertheless, when the spindle is rotated at a high speed, portions of the bearing are not sufficiently cooled by the liquid lubricant which passes through the radial passageways of the inner race, and thus the thermal expansion among portions of the bearing is different and thus the accuracy of the roundness of the rolling tracks of the inner and outer races is reduced. As a result, various defects, such as an increase of noise during the running of the spindle bearing and a shortening of the operating life of the spindle bearing, appear.
Moreover, when the liquid lubricant is supplied into the cavity of the spindle rotated at a high speed, a piping element is used for introducing the liquid lubricant into the cavity of the spindle. Namely, an end of the piping element is inserted into an opening of an end of the spindle via a small radial gap therebetween, to prevent the piping element from coming into contact with the spindle, and the cavity of the spindle is maintained in a negative pressure state to thereby permit air to enter the cavity of the spindle from the atmosphere via the small air gap, to thus prevent a flow of the liquid lubricant out of the cavity of the spindle toward the outside thereof through the small air gap. Namely, an air seal is formed between the piping element and the spindle opening, and prevents a seizing of the piping element when the spindle is rotated at a high speed. When the spindle is rotated at a high speed, the liquid lubricant supplied by the piping element into the interior of the spindle is urged toward and attached to the inner wall of the cavity of the spindle, under a centrifugal force, but on the other hand, the air entering the interior of the spindle remains there as a mass of air. At this stage, when considering the distribution of the liquid lubricant within the cavity of the spindle, i.e., in the axial bore of the spindle, it can be understood that such a lubricant distribution is complicatedly varied in response to a change in an amount of the liquid lubricant supplied to the bearing through respective radial passageways of the spindle under a centrifugal force. Particularly, when a rotating speed of the spindle is increased, an axial distribution of the liquid lubricant in the axial bore of the spindle varies greatly. For example, when a spindle is supported by two bearings located at a position A adjacent to the end of the lubricant supply piping and another position B far from the end of the lubricant supply piping, the liquid lubricant distributed in the axial bore of the spindle at a position near to the above-mentioned position A is substantially equal to that at a position near to the above-mentioned position B, during the rotation of the spindle at a low speed. When the rotating speed of the spindle is increased, however, the distribution of the liquid lubricant at the position near to the position A is reduced, and the distribution of the liquid lubricant at the position near to the position B is greatly increased. Accordingly, the bearing supporting the spindle at the position A is not sufficiently lubricated compared with the bearing supporting the spindle at the position B, and thus a seizing of the former bearing occurs due to a lack of lubrication.